To ISO, or not to ISO

[Scot Yount]

I am hoping you guys can help me clear something up regarding ISO settings on RED Cameras. RED's own literature makes distinctions with respect to ISO on its website, essentially saying that your ISO setting during shoot doesn't really matter in terms of what is recorded as RAW data. It does however, give you a "starting point" of sorts with which to make many other aesthetic and technical choices while shooting.

From the website:
".....With other cameras, a higher ISO typically means the signal gets amplified and filtered before being recorded—effectively baking ISO into the RAW data.....With RED, the original signal remains unamplified before being saved as a RAW file. The idea is to record as much as possible from the sensor, and to maximize flexibility for adjusting exposure in post-production. As a result, the full 13+ stop dynamic range is captured regardless of ISO choice, and ISO can be specified after the exposure. ISO only affects how the RAW data is interpreted."

My friend, whom I respect a great deal, is a longtime commercial still photographer, (An AFI grad, started in cinematography, worked on Pulp Fiction and other big films back in the day and now shoots background plates for many auto makers, including Mercedes Benz) tells me that the info on the website regarding ISO, is essentially marketing bunk. He says increasing the ISO on his Nikon D800 charges the sensor more, making it more sensitive to light and visa versa. I was pretty sure that both RED and other cameras have amplifiers that accomplish the task of ISO (a reasonable facsimile) after the sensor, and that his camera "bakes" that setting in, and mine doesn't. Can anyone explain this better?

signed,
Confused. Sort of.

 

[Marc Wielage]

As far as I know, the ISO is just part of the metadata with the file and can be dynamically changed later on. The raw file is just a whole image capture right off the sensor. If you change the T-Stop of the lens, that will absolutely change the exposure that's baked in to the file. ISO you can change after the fact; the actual lens exposure you can't.

My personal preference is for clients to generally underexpose a little bit (maybe 1/3-1/2 a stop) in order to avoid blowing out the highlights. In dark scenes, I request (politely) that they change the key-to-fill ratio to fill the shadows reasonably, because we can always crush it later on. Camera tests are often necessary to figure out how the camera is going to respond under different conditions.

 

[Nick Morrison]

Yes, raising ISO "does" amplify the signal and noise. No one's disputing that. However the value of shooting RAW (vs say QT video) is you can change your mind about ISO (and color temp) after the fact. Most stills photographers regularly shoot RAW stills, so this is nothing new for them.

However, in video....RED's compressed RAW workflow was nothing short of revolutionary, and remains so. Most other camera platforms now provide a RAW workflow, but it's usually much less compressed and quite data heavy.

However, just because you can change your ISO with RAW....doesn't mean you shouldn't get your EXPOSURE right in the first place. The sensor is basically recording whatever you tell it to record - no manipulation after the fact will magically "add" more light. It wont. So you still need to learn how to "rate" your RED, and feed it enough light. That said, having the extra flexibility of RAW in post is still a MAJOR boon.

Hope this helps.
best

 

[Scot Yount]

Thanks Marc,
I completely understand why I want to choose an ISO, so that the elements you are talking about, like T-stop, shutter speed/angle that you can't change later obviously are properly chosen while shooting. Just common sense. At least that is what I thought.

 

[Scot Yount]

Yes, raising ISO "does" amplify the signal and noise. No one's disputing that. However the value of shooting RAW (vs say QT video) is you can change your mind about ISO (and color temp) after the fact. Most stills photographers regularly shoot RAW stills, so this is nothing new for them.

Right, but the question is whether that ISO is baked into the .r3d files or not. According to RED's literature, it isn't, but IS on other cameras. Sensors are sensors. Light hits them, they make current. More light, more current, less, less. Changing ISO is changing an amplifier AFTER the sensor right? So, in a Nikon D800 or a Canon 5D is that amplification baked into the RAW data or not. RED says (unless I am reading it wrong) that the ISO setting is just metadata. My friend is adamant that it isn't. When he saw the slider to change ISO in RCX, he denounced it as bunk marketing jargon. Uh......Of course you couldn't in film. But unless I am wrong, sensors don't have ISOs. It is just a way of understanding what used to be film speed and is now electronic amplification.

 

[Gunleik Groven]

Well they are both right...

Shooting RED, the ISO is only metadata.
On some (not all) other cameras it isn't.

ISO is however "baked" when it is done through metadata, but not reversible in post.

To me the choice of ISO approach isn't really a "moral one".

The important thing is to learn how it works in your tool and make it work right for you.

 

[Nick Morrison]

Right, but the question is whether that ISO is baked into the .r3d files or not. According to RED's literature, it isn't, but IS on other cameras. Sensors are sensors. Light hits them, they make current. More light, more current, less, less. Changing ISO is changing an amplifier AFTER the sensor right? So, in a Nikon D800 or a Canon 5D is that amplification baked into the RAW data or not. RED says (unless I am reading it wrong) that the ISO setting is just metadata. My friend is adamant that it isn't. When he saw the slider to change ISO in RCX, he denounced it as bunk marketing jargon. Uh......Of course you couldn't in film. But unless I am wrong, sensors don't have ISOs. It is just a way of understanding what used to be film speed and is now electronic amplification.

Ummm...it's not random marketing junk.

If you shoot Prores, your ISO choice is LOCKED. Yes you can adjust this in post, but at the expense of DR (ie as you boost or lower the signal, your signal clarity starts to degrade). When shooting RAW, you can change ISO non-destructively and "shift" your DR up and down (ie more towards highlights, or more towards shadows). Tell your friend its the same as a DSLR shooting "RAW" vs "JPEG". Shooting QT is equivalent to shooting jpeg - color and exposure choices are more limited in post.

So RAW workflow doesn't change your exposure - it simply gives you more flexibility over how to manage it in post.

And this is not a "RED" thing. Alex, Sony, Canon and Blackmagic all shoot flavors of "RAW".

"RAW" is a becoming more popuar on high-end production these days (again not something specific to RED). RED is simply unique in that they were very early proponents of RAW, have been offering compressed RAW since inception, and until very recently with Weapon (which can also shoot Prores)...have championed an all RAW workflow since the first RED ONE-M.

 

[Akin A]

Scot, I'm assuming you are referring to the RED 101 article in your original quote. That's a good place to start, followed by reading this entire thread:
http://www.reduser.net/forum/showthread.php?126194-Native-Dragon-ISO%85once-and-for-all

+1 to what Nick said.

If you want a simple way to look at it, when recording raw, ISO is just a note (metadata) to your NLE (and your camera's monitor) to pull up or down on the mid-tones on a special exposure curve. The whitest whites stay put and the blackest blacks stay put. The mid-tones have the most drastic change in exposure and the change is gradually less towards the black and white extremes. ISO has nothing to do with what's actually recorded by the sensor, other than the fact that you'll likely change your lighting, aperture, ND filter, and maybe shutter, based on how things look at the selected ISO.

 

[sander kamp]

RED doesn't apply amplification at the sensor level as other manufacturers do. Instead they apply a curve to the preview image to raise the midtones and shadows to match a ISO standard. The good thing about that is that you can change ISO in post and the highlights are not clipped at higher ISO's, the downside might be a bit more noise in the shadows.

I tend to think of ISO on RED cameras more as a brightness level, it will give you a preview of what will happen if you apply a certain curve to your image in post.

Let's say you have a well lit scene without any hot spots and you want to expose to the right to minimize noise in the image, in that case I would open up the iris and lower the ISO to 250 or 320 to get a darker preview. On the other hand if you a high contrast scene and you want to avoid clipping the high lights you close your iris and set the ISO to 800 (or even higher with the low light OLPF) to get a brighter preview.

Best to do some testing to find out how much noise is still acceptable for your purpose and then use that ISO level as a maximum.

 

[Marc Wielage]

If you shoot Prores, your ISO choice is LOCKED. Yes you can adjust this in post, but at the expense of DR (ie as you boost or lower the signal, your signal clarity starts to degrade). When shooting RAW, you can change ISO non-destructively and "shift" your DR up and down (ie more towards highlights, or more towards shadows). Tell your friend its the same as a DSLR shooting "RAW" vs "JPEG". Shooting QT is equivalent to shooting jpeg - color and exposure choices are more limited in post.

Not really. I don't think there's even 5% difference between shooting ProRes 444 Log and Raw -- provided the baked-in color temperature and basic exposure are correct. I've done many, many Red and Alexa projects where we went to ProRes 444 for the final color pass, and it was fine. I had more than enough range to get what I needed out of the signal. When I didn't, we'd go back to the Raw, change as needed, and re-render those ProRes files.

In general, if you screw up the Raw exposure badly enough, you still can't get any additional detail out of it. Even Raw can only go so far -- once the image highlights are badly clipped, it's clipped forever. We can noodle it to some degree, but it's basically putting a little perfume on a turd. Exposed properly, either Raw files or ProRes files look fine.

Given infinite time and money, I'd go for the Raw files every time. But we often don't have that luxury, so going with ProRes 444 files is fine -- provided the transcode is done precisely, paying close attention to the DPs settings in the original files, and the ISO, Exposure, Tint, and Color Temperature are all preserved. (In truth, ISO and Color Temperature are by far the most important; the others I can match within most color-correction programs.)

It's an interesting comparison to see how little difference there is between an ISO adjustment in the Raw file vs. just a Gain adjustment in the color correction controls. I don't see any additional noise either way -- they're pretty much identical.

 

[Nick Morrison]

Not really. I don't think there's even 5% difference between shooting ProRes 444 Log and Raw -- provided the baked-in color temperature and basic exposure are correct.

Agreed. I just happen to have worked on enough projects where iso and color temp weren't always perfect (imagine a reality promo shoot filled with mixed lighting and wildly divergent exposures btw takes)...and shooting RAW...and being able to ADJUST these choices later in the grade...was immensely valuable.

If you shoot accurately, I agree...RAW loses much of it's value. But when you need to sometimes punt those choices for later, it's a fantastic way to work.

 

[Gunleik Groven]

This is a two edged sword...

You should always expose well for the result you want.
That will always give you the best result.

RAW just gives you a bit more of leeway, if that isn't the case (which it often isn't).
RAW does NOT mean that that underexposed shot will be noise-free, though. It just means that you perhaps actually can use it.

It's not really a crutch to solve your lacking as a photographer, but if you shoot it right, it can get you to "better", and if you shoot it wrong it can get you to "solveable".

RAW does not, however, make you a DoP.

 

[Dr. Sassi]

Please share the source of the ISO of the D800. I can't find it anywhere. Not on Nikon's website nor on pages like
http://www.dpreview.com/reviews/nikon-d800-d800e/24
I would highly appreciate anything, seriously... I don't ask to troll here around. It is the first time I read that something can create higher sensor charges just with a push of a button, even if the light that hits the sensor is equal or less. Thanks in advanced, and I hope it is OK to ask, as it was part of the initial statement.

 

[Nick Morrison]

It's not really a crutch to solve your lacking as a photographer, but if you shoot it right, it can get you to "better", and if you shoot it wrong it can get you to "solveable".

RAW does not, however, make you a DoP.

Exactly.

 

[David Battistella]

I look at it this way.

Lets say your our friend shot 300 photographs on his d800 at 800 ISO in raw. When he opens those photos in his favorite photo editor, he has the option to change many of the settings including amplification of the signal. Maybe using the exposure tool in raw, he could bring back highlights a bit and move that image aroumd. RAW images are more "elastic", contain more data and how that data is extracted can be changed or improved over time. His ISO choice would remain with the file but many other parameters could be moved. ISO could also be telling his post software how to process the raw.

Generally, an ISO rating or selection is an exposure guideline and the choice you make in ISO will affect your final image especially when it comes to grain. In stills on a Nikon this is a bit different because as I adjust the ISO setting, the camera manufacturer can tell the camera to adjust values based on that ISO setting. For example, switching to a higher ISO might trigger noise reduction that is baked into the file. It's makes sense for stills because it is doable and requires far less processing power but it's easier to apply that power to a single image rather than an image stream.

So a d800 plays with your pixels when you set higher ISO's, that might mean they desaturated, slightly crush blacks, use nr, sharpen or what no to give you the file. Then you can further raw process the file with their tools, but ISO is not a value you move around because it is generally used as a basis for exposure and metering.

This is also true for RED, ISO should be used as a basis for exposure and metering. As an example. I can meter and rate scenes at 250 ISO or 1600 ISO and choosing either of those ratings will affect my shutter angle and aperture, those are ultimately the only two things that affect the amount of light hits the sensor and then gets written to a RED file. Or, put another way, if I rated a scene higher or lower, i might move the aperture. This is a clear example of how ISO affects exposure on RED. For example, I just shot a commercial at 1000 ISO because I wanted deeper focus look and I used the ISO rating to be able to close down and attain a stop value and deeper focus.

The difference is in processing and signal to noise on the output file. With RED, I could use exposure tool, like your friend, when processing RAW or I could choose ISO to perform the same task of raising or lowering base exposure and BOTH would have an effect on grain.

In a sense, you friend is absolutely correct because when YOU, the cinematographer, rate the camera at 1600 ISO, you have, in a sense baked-in, NOT THE ISO BUT, the amount of light hitting the sensor into the file. You can never change that, so if you underexposed and have a grainy file, no amount of messing with ISO is going to save your negative. (But neatvideo might)

The key is discovering an exposure that gives you the kind of flexibility you want and always using the quality and quantity of light you need or want to create your image.

One thing I have noticed is that with properly exposed images (when I say this I mean that the photosites are receiving enough light to create a generally noise free elastic image), your ISO post choice can really improve your images.

Last week I was looking at some really clean well exposed images exposed at 320 ISO, but the histogram was sitting a bit left and it was creating some shadow noise, moving the ISO to 1000 flattened the histogram and moved those shadow values up and there was less noise in that critical part of the image and more room to create a pleasant image without sacrificing the whole shot. That is where moving ISO in post can help, but your friend might have accomplished the same thing with the exposure slider. If the shot had been rated at 1000 to begin with I might not have that latitude or flexibility to change things around. This is also known as capturing a meaty or fat or thick negative, where the photosites are not light starved.

So I see both sides. What is important about the base ISO is to understand that this is something YOU do actually choose and that choice does have an impact on your final image and that exposure ISO rating does especially have an effect if it causes you to change the amount of light hitting the sensor (with shutter or aperture).

ISO on RED can also be used purely as a monitoring tool. In other words, I expose everything at 500ISO and light for an aperture setting and shutter angle, but then on the camera, I set the ISO to 1000 just to see things a bit better, knowing that I will process everything at 500 ISO, that is why we say it's not baked-in, because I can use ISO for monitoring purposes instead of exposure. Maybe, it's easier to pull focus at 1000 ISO, but I have still rated at 500 and I can quickly reset the files to 500ISO in RCX.

 

[Graeme Nattress]

"tells me that all that "bunk" on the website is just that. BS. He says increasing the ISO on his Nikon D800 charges the sensor more, making it more sensitive to light and visa versa". As for as the RED site goes, how we do ISO is accurately and correctly explained.

Some cameras use a analogue gain to accomplish ISO gain. We don't. Some cameras use analogue and digital gain to accomplish ISO, and some will even "burn in" the digital gain into the raw signal that gets recorded. In all cases, burning in gain changes to the raw signal, be that analogue or digital gain (or some combo of both) will clip at the top end and reduce dynamic range.

I know of no mechanism whereby the light sensitivity of a sensor can be altered on the fly like your friend is describing. It would be a fantastic thing to have though, eliminating the need for ND over-night, which of course hasn't occurred because this technology doesn't exist.

Graeme

 

[Björn Benckert]

As Grame write, no sensor can boost light. But many cameras can gain the signal and save that as the result. So RED leaves this option open / saved ISO as meta data together with the raw file captured.